In OFDM based transmission systems, the number of data bits carried by one OFDM symbol (a type of layer 1 frame) tends to be very large compared to that of conventional low speed transmission systems. For example, if one OFDM symbol consists of 1024 sub-carriers with QPSK modulation and rate one half channel encoding, and 80% capacity used for data bits, the number of data bits which would be transmitted for a single OFDM symbol would be about 100 bytes. If channel conditions allow, the number could be even as high as 800 bytes with the 256 QAM modulation scheme and near rate one coding. With these high-speed data transmission schemes, it is important to have an efficient data-packing scheme because even a fraction of the symbol that is not used for transmission could end up being several hundred bytes long.
It has become common to have variable length layer 2, or MAC frames, in order to support the bursting nature of data arrivals. It is desirable to have layer 2 frames which are capable of carrying complete upper layer transport units, for example IP packets. Thus, rather than forcing the layer 2 frame to be a fixed multiple of the layer 1 frame length, it has become common to allow the layer 2 frame to have a variable length thereby avoiding the potential wastage which would occur in the event that the layer 2 frame was a fixed size and the upper layer packet does not fit into the layer 2 frame.
In a variable length layer 2 frame, the length information is located in the header of the layer 2 frame so the receiver knows the end of the frame as well as the beginning of the next frame. Disadvantageously, when the header is in error, the receiver does not know where the next frame starts and consequently will lose data in subsequent layer 2 frames until a new layer 2 frame boundary is detected. In one effort to deal with this issue, layer 1 frames are grouped in “super-frames” for example consisting of 10 layer 1 OFDM symbols. The beginning of such a super-frame is always used to start the transmission of a new layer 2 frame. Thus if a header is in error and the receiver is unable to determine the next layer 2 frame boundary, the receiver will always know that there will definitely be a layer 2 frame boundary at the start of the next super-frame. Unfortunately, this can result in a substantial loss of data.
An example of this is shown in FIG. 1. In FIG. 1 a sequence of layer 1 OFDM symbols is indicated by 10. Two scheduling intervals 11 and 12 are shown, these being equivalent to the above referenced super-frames. A MAC frame 12 starts at the beginning of the first scheduling interval 11, and similarly a MAC frame 18 starts at the beginning of the second scheduling interval 12. Scheduling interval 11 also includes MAC frames 14 and 16 transmitted in sequence after MAC frame 12. MAC frames 12, 14 and 16 have respective headers 13, 15 and 17. Similarly, during the second scheduling interval 12, MAC frames 20, 22, 24 and 26 follow MAC frame 18, and the MAC frames 18, 20, 22, 24 and 26 have respective headers 19, 21, 23, 25 and 27. In the event during reception over a scheduling interval 11, MAC frame 14 is received with an error in its header 15, the system is unable to determine the length of MAC frame 14 and as such does not know where the start of the next MAC frame 16 will be. For this reason the entirety of MAC frames 14 and 16 will be lost, and the receiver can only start decoding MAC frames again beginning with MAC frame 18 because it knows that a new scheduling interval 12 begins there because that is the start of a super-fame. A similar loss is shown during scheduling interval 12 in which there is shown to be an error in the header 21 of MAC frame 20 resulting in the loss of the entirety of MAC frames 20, 22, 24 and 26. In OFDM based very high data rate transmission systems, these types of loss can result in severe performance degradations since the number of data bits in one symbol is large and each header carries other sensitive control information.
Another existing method of trying to deal with this issue is to include a predetermined bit pattern as a frame start and frame end indication. Such start and end indicators may require more bits than the original header information, and are also subject to error.